Certain modern adhesives and sealants (hereinafter, collectively referred to as sealants) are formed from two components that are mixed just prior to use. For example, a polysulfide-type sealant is formed from a mixture of a base material, such as sodium polysulfide, and a catalyst, such as an organic dichloride.
The base and catalyst must be mixed in a precise ratio so that the resultant sealant cures properly. Accordingly, the pumps that deliver the two sealant components to the mixing mechanism must be precisely controlled to deliver the correct ratio of catalyst to base.
The base and catalyst may be delivered to a dispensing gun. The gun includes a housing that contains one or more mixing chambers. The base and catalyst are combined and mixed as they flow through the mixing chambers. The mixed components (i.e., sealant) are dispensed through a nozzle that is connected to the housing. Flow of the base and catalyst through the mixing chambers is controlled by valves that are actuated by a lever or trigger mechanism that is manually operated by the gun user.
Some metering systems are designed to mix the pumped base and catalyst in a single mixing conduit that extends between the pumps and the dispensing gun. The dispensing gun may or may not have additional internal mixing chambers. Metering systems that mix the components within a long conduit are expensive to operate because the mixing conduit must be purged whenever the sealant dispensing operation is halted for a significant period of time. Purging is typically carried out by forcing only the base material through the conduit and gun. Accordingly, the large amount of the sealant and base used for purging is lost each time the dispensing operation is halted.
Systems that mix components only within the dispensing gun waste less sealant, since only the relatively small volume of the mixture within the gun must be purged each time the operation is halted. It is noteworthy, however, that the overall amount of component mixing carried out within such a system is generally less than the mixing provided by a system that mixes within both a gun and a delivery conduit. Moveover, as the amount of component mixing decreases, it becomes increasingly important to provide the precise specified ratio of catalyst to base in order to obtain satisfactory curing of the sealant. In short, a metering system that mixes components only within the dispensing gun must have reliable and accurate mechanisms for consistently providing catalyst and base in the specified ratio. Further, the metering system must include fine adjustment mechanisms for easily and quickly adjusting the ratio as necessary to account for changes in the mixing or curing characteristics of the base or catalyst.